The virus-like particles located in intracisternal spaces of tumor cells of the L2C transplantable guinea pig leukemia will be further characterized by a variety of biochemical, biophysical and immunological methods. The properties of this virus, which appears to accumulate in the plasma of leukemic guinea pigs, will be compared to those of the BrdU- inducible B type guinea pig virus. This latter virus will also be studied in this laboratory to determine if this endogenous virus is related to mouse mammary tumor virus as measured by immunological cross reactivity or hybridization.